While not limited thereto, the present invention is particularly adapted for use in liquid crystal welding helmet lens assemblies such as those shown in U.S. Pat. Nos. 4,039,254 and 4,039,803. Such a protective welding lens assembly comprises a layer of nematic liquid crystal material sandwiched between opposing parallel plates coated with transparent conductive films. The faces of the plates in contact with the liquid crystal material are treated so as to effect a twisted nematic structure therein which will rotate polarized light passing through the light shutter through 90.degree.. Polarizers are disposed on opposite sides of the plates such that by applying an electrical field across the transparent conductive films, the opacity of the liquid crystal light shutter can be changed.
It is preferable in a welding helmet lens assembly to utilize parallel polarizers on opposite sides of the liquid crystal cell such that, when no electric field is applied across the transparent conductive films, polarized light cannot pass through the cell. That is, in order to render the cell light-transmitting, an electric field must be established across the conductive films. Among other things, this is for the reason that in the event of power failure, the cell will automatically assume an opaque condition and protect the eyes of the welder.
During a welding operation, the welding arc is sensed by a phototransistor or other wave energy sensing device which, through suitable circuitry, removes the electric field across the liquid crystal layer, thereby causing the cell to become opaque. It is, of course, highly desirable to switch from the light-transmitting to the opaque condition as quickly as possible. Nevertheless, because of the speed of operation of liquid crystal light shutters of this type, the welder will observe a momentary flash during the time the cell is changing from a light-transmitting to an opaque condition upon the establishment of a welding arc.
The momentary flash which the welder observes when an arc is initially struck presents special problems in the cause of aluminum welding. Electric welding of aluminum requires techniques different than those for conventional steels. That is, aluminum conducts heat very rapidly; and the weld bead head cools quickly when the electrode is removed from the work. As a result, rapid cooling results in weak welds. In order to overcome this condition, it is common practice to place a short weld at the end of the weld head two or three seconds after welding the main head. This slows chilling of the head and produces high quality welds. Because of the speed of operation of the liquid crystal light shutter as explained above, a welder working with aluminum will receive a momentary flash of light when he applies the welding electrode after two or three seconds to reheat the rapidly-cooling bead; and it is, of course, desirable to eliminate this condition.